In order to verify the possible necessity of replacing a lamp in public lighting installations, reliance is generally placed on direct observation either by teams of monitoring staff or by private citizens who take it upon themselves to notify faults to the network management authority.
In addition to this, so-called `remote monitoring` systems have been available for some time which comprise an electronic network to sense the state of operation of the individual lamps. All the information collected on an entire lighting network is then directed to a single central monitoring station. Systems of this type are described, for example, in patent documents EP-A1-0347317, FR-B1-2592718, FR-A1-2646581, DE-A1-3635682, U.S. Pat. No. 4,939,505, IT-B-1227507, IT-B-1229228.
The above-mentioned systems very in the manner in which they sense whether the lamp is on or off. In particular, in some examples, monitoring is based on current sensing (IT-B-1227507, IT-B-1229228), in others on sensing the voltage at the lamp terminals (IT-B-1229228 again), in others on sensing the luminous flux (FR-B1-2592718), and in others on sending test signals (U.S. Pat. No. 4,939,505, EP-A1-0347317). The system described in FR-A1-2646581 uses current sensing to determine whether the lamp is on, but a fault signal is not sent until it is verified that an appropriate voltage is present; this prevents drops in line voltage from causing generalized signaling of non-existent faults.
It has, however, been found that lamp failure is almost never an unexpected phenomenon. In fact, emission of light progressively decreases as the lamp ages. Indeed, in some types of gas discharge lamps complete failure is preceded by a period of intermittent operation, during which the functionality of the lamp may be considered to have come to an end, although current and voltage values do not deviate significantly from those of efficient lamps.